The present invention to a first aspect relates to a method and a system in an ATM system for controlling flows of data cells and flow control management cells from a number of sources to a destination over connections passing a network element, while returning the flow control management cells from the destination via the network element to the respective sources. The network element may be exposed to congestion due to contention between the connections, which necessitates queuing of the connections. The data cells include lower priority cells and higher priority cells. The flow control management cells have an explicit rate field for an explicit rate value used to limit a source maximum allowed cell rate to a specific value, and a current cell rate field for receiving said specific value.
According to a second aspect the invention relates to a method and a system in an ATM system for controlling flows of data cells and flow control management cells from a number of sources to a destination over connections passing a respective input buffer and a common output buffer of a fair queuing switch, while returning the flow control management cells from the destination via the switch to the respective sources. The switch is exposed to congestion due to contention between the connections, that necessitates queuing of the connections in the input buffers and output buffer. The flow control management cells have an explicit rate field for an explicit rate value used to limit a source maximum allowed cell rate to a specific value, and a current cell rate field for receiving said specific value.
The use of ATM as a new and overall solution for data communication, spanning from the local area to the wide area has proven to be burdened with a significant amount of problems. Most of these problems are more or less inherently connected to the properties of the data communication traffic as such, which to a great extent differ from characteristics well known from the telecommunications domain.
In general, applications using data communication services require large amount of bandwidth during rather short periods of time, causing offered traffic with bursty characteristics. Moreover, the information sent between typical computer applications must be error free, but may be affected by some transfer delay without deteriorated performance. Applications within the telecommunications domain could be said to have the opposite characteristics, i.e. the bandwidth is kept constant and they are not too sensitive to bit errors but sensitive to delays and variation of delays.
Consequently, data communication traffic must be managed differently from the telecommunication traffic within the ATM networks if the trade-off between network utilization and quality of service (QoS) is to be kept at an acceptable balance.
Recent efforts within standardization for a reflect the need for a specific ATM service to handle traffic with "pure" data communication properties. A service called the Available Bit Rate ABR service is specified in ATM Forum Traffic Management Specification 4.0, ATMF 95-0013R10, Feb. 1996. This service will be included in the ITU-T recommendations, as well, cf. e.g. ITU-T Recommendation I.371, "Congestion Management for the B-ISDN", 1995.
The ABR service utilizes a rate based congestion control concept. The network controls the rate at which the users mate transmit data by means of feedback information sent to the sources.